1. Field of the Invention
The present invention relates to a modulation circuit for laser diode. More particularly, it relates to a low frequency square wave modulation circuit for laser diode which operates in a laser diode driving circuit to regulate the intensity of the laser output. Hereinafter the aforesaid driving circuit is called an APC circuit which is composed from the devices of; sampling a fraction of light emitted from the laser diode, controlling a driving current to be input to a diode driver for laser diode by output signal from the aforesaid sampling device in order to regulate an intensity of the laser output.
2. Decription of the Prior Art
FIG. 2 illustrates a sample of a block diagram of a square wave modulation circuit in the prior art. It is composed from a photo detector (1), a current-voltage converter (2), a differential amplifier (3), a reference voltage generator (4) a smoothing circuit (5), a square wave generator (6), a diode driver (7) and a laser diode (8).
FIG. 2 is also at the same time an APC circuit which has the function to regulate the output intensity of laser diode (8) at a constant level.
The photo detector (1) samples a fraction of light emitted from the laser diode (8), an output current from aforesaid photo detector (1) is converted to voltage by the current-voltage converter (2) which, in turn, is inputted to the differential amplifier (3).
For example, when an output intensity of laser diode (8) is 1 mW, the photo detector generates a current of about 0.1 mA through sampling.
The output of the reference voltage generator (4) is applied to the other terminal of the differential amplifier (3). Then the differential amplifier (3) boosts the difference between the output of the reference voltage generator (4) and output voltage of the aforesaid current-voltage converter (2) in order to supply an output signal to a diode driver (7) through a smoothing circuit (5).
For example, the differential amplifier (3), with the reference voltage generator (4) of 0.8 V, may satisfy the conditions below; the output of differential amplifier (3) is -8 V when an input voltage of 0.8 V is applied from the current-voltage converter (3), it becomes to -9 V when the input voltage is changed to 0.9 V, and the output of said differential amplifier (3) varies to -7 V when it receives the input of 0.7 V from current-voltage converter (2). Thus, the output of differential amplifier increases or decreases according as the corresponding output of current-voltage converter (2) decreases or increases.
The diode driver (7) feeds the driving current to the laser idode (8).
FIG. 2 illustrates a feedback loop comprising laser diode (8), photo detector (1), current-voltage converter (2), differential amplifier (3), smoothing circuit (5), diode driver (7) and laser diode (8) to regulate the output intensity of laser diode (8).
The output intensity of laser diode (8) becomes stable according as the output voltage from the current-voltage converter (2) gets close to that of the reference voltage generator (4), and it is most stable when the two voltages get equivalent.
A square wave generator (6) is connected to the smoothing circuit (5) in order to impose a square wave modulation on the photo output of the laser diode (8).
The smoothing circuit (5) with a large time constant is required for the feedback loop not to generate the photo output by the APC circuit when the square wave photo output is "OFF" in FIG. 2.
The aforesaid smoothing circuit (5) is composed of an integrating circuit and it integrates input signal to generate the photo output square wave with a level of the mean value of the input signal.
According as a frequency of the square wave becomes lower, a larger time constant is required. As a matter of fact, due to the marginal value of applicable time constant, a problem arises that it is difficult to perform the square wave modulation at low frequency such as about 270 Hz, in such a circuit shown in FIG. 2.